Erasing method for 3d nand flash memory

ABSTRACT

Embodiments of erasing methods for a three-dimensional (3D) memory device are disclosed. The 3D memory device includes multiple decks vertically stacked over a substrate, wherein each deck includes a plurality of memory cells. The erasing method includes checking states of the plurality of memory cells of an erase-inhibit deck and preparing the erase-inhibit deck according to the states of the plurality of memory cells. The erasing method also includes applying an erase voltage at an array common source, applying a hold-release voltage on unselected word lines of the erase-inhibit deck, and applying a low voltage on selected word lines of a target deck.

CROSS-REFERENCE TO RELATED APPLICATIONS AND INCORPORATION BY REFERENCE

This application is a continuation of U.S. patent application Ser. No.17/062,053 filed on Oct. 2, 2020 and titled “Erasing Method For 3D NANDFlash Memory,” which claims priority to PCT/CN2020/111689 filed on Aug.27, 2020, both of which are incorporated herein by reference in theirentirety.

TECHNICAL FIELD

The present disclosure generally relates to the field of semiconductortechnology, and more particularly, to an erasing method for athree-dimensional (3D) memory.

BACKGROUND

As memory devices are shrinking to smaller die size to reducemanufacturing cost and increase storage density, scaling of planarmemory cells faces challenges due to process technology limitations andreliability issues. A three-dimensional (3D) memory architecture canaddress the density and performance limitation in planar memory cells.

In a 3D NAND flash memory, many layers of memory cells can be stackedvertically such that storage density per unit area can be greatlyincreased. To further increase storage density, multiple decks can bestacked vertically, where in each deck there are many vertically stackedmemory cells. To read, write and erase efficiently in a 3D NAND flashmemory with multiple decks, each deck can be treated as a separatememory block, i.e., each deck can be erased independently from the otherdecks. However, the erase operation of a selected deck can be affectedby the neighboring decks that are connected in serial. For example, whenerasing a top deck, depending on the data stored in other neighboringdecks, charge carriers for the erase operation may not be able totransport to the selected deck. Accordingly, the selected deck may haveerase-fail bits due to the influence from neighboring decks. Therefore,a need exists for a method to erase a selected deck in a 3D NAND memoryso that each memory cell in the selected deck can be reset to the erasedstate accurately and efficiently.

BRIEF SUMMARY

Embodiments of a method for an erase operation in a three-dimensional(3D) memory device with multiple decks is described in the presentdisclosure.

One aspect of the present disclosure provides erasing methods for athree-dimensional (3D) memory device, that includes multiple decksvertically stacked over a substrate, where each deck includes aplurality of memory cells. The erasing method includes checking statesof the plurality of memory cells of an erase-inhibit deck and preparingthe erase-inhibit deck according to the states of the plurality ofmemory cells. The erasing method also includes applying an erase voltageat an array common source or an array cell well-body, applying ahold-release voltage on unselected word lines of the erase-inhibit deck,and applying a low voltage on selected word lines of a target deck.

In some embodiments, preparing the erase-inhibit deck includes applyinga first preparation voltage on the unselected word lines of theerase-inhibit deck when the plurality of memory cells of theerase-inhibit deck are in programmed states.

In some embodiments, applying the first preparation voltage includesapplying a voltage between about 0 V and about 1 V.

In some embodiments, applying the first preparation voltage includesapplying 0 V.

In some embodiments, preparing the erase-inhibit deck includes applyinga second preparation voltage on the unselected word lines of theerase-inhibit deck when the plurality of memory cells of theerase-inhibit deck are in erased states. The second preparation voltageis larger than the first preparation voltage.

In some embodiments, applying the second preparation voltage includesapplying a voltage between about 1 V and about 7 V.

In some embodiments, preparing the erase-inhibit deck includes applyinga first preparation voltage on a first subset of the unselected wordlines of the erase-inhibit deck when a first subset of the plurality ofmemory cells having the first subset of the unselected word lines are inprogrammed states, and applying a second preparation voltage on a secondsubset of the unselected word lines of the erase-inhibit deck when asecond subset of the plurality of memory cells having the second subsetof the unselected word lines are in erased states. The secondpreparation voltage is larger than the first preparation voltage.

In some embodiments, the second subset of the plurality of memory cellsincludes at least two neighboring memory cells.

In some embodiments, preparing the erase-inhibit deck includes applyinga first preparation voltage and a second preparation voltage that islarger than the first preparation voltage, on the unselected word linesof the erase-inhibit deck randomly, when a first subset of the pluralityof memory cells in the erase-inhibit deck are in programmed states and asecond subset of the plurality of memory cells in the erase-inhibit deckare in erased states.

In some embodiments, applying the low voltage comprises applying avoltage in a range between about 0 V and about 1 V.

In some embodiments, applying the low voltage comprises applying avoltage of 0 V.

In some embodiments, applying the hold-release voltage includes applyinga voltage of 0 V, and subsequently removing the voltage of 0 V and anyexternal bias.

In some embodiments, applying the erase voltage includes applying avoltage in a range between about 15 V to about 25 V.

In some embodiments, applying the erase voltage includes applying about20 V.

In some embodiments, the erasing method also includes applying the erasevoltage at bit lines of the target deck.

In some embodiments, the erasing method further includes applying thehold-release voltage on the selected word lines of the target deck priorto applying the low voltage on the selected word lines of the targetdeck.

In some embodiments, the erasing method further includes applying thehold-release voltage on unselected word lines of a bottom deck, andapplying the low voltage on selected word lines of a top deck. The topdeck is vertically stacked on the bottom deck over the substrate.

In some embodiments, the erasing method further includes applying thehold-release voltage on unselected word lines of a top deck and a bottomdeck, and applying the low voltage on selected word lines of a middledeck. The top deck is vertically stacked on the middle deck and themiddle deck is vertically stacked on the bottom deck over the substrate.In some embodiments, the erasing method further includes applying theerase voltage at bit lines of the top deck.

Another aspect of the present disclosure provides a three-dimensional(3D) memory device having multiple decks vertically stacked over asubstrate. Each deck includes a film stack of alternating conductive anddielectric layers, and a plurality of memory strings penetrating throughthe film stack, where each memory string includes a plurality of memorycells. The 3D memory device also includes a conductive plug disposedbetween adjacent decks, electrically connecting the plurality of memorystrings between the adjacent decks. The multiple decks of the 3D memorydevice includes a target deck and an erase-inhibit deck. The 3D memorydevice is configured to erase the target deck by the following steps:checking states of the plurality of memory cells of the erase-inhibitdeck, preparing the erase-inhibit deck according to the states of theplurality of memory cells of the erase-inhibit deck, applying an erasevoltage at an array common source or an array cell well-body, applying ahold-release voltage on unselected word lines of the erase-inhibit deck,and applying a low voltage on selected word lines of the target deck

Other aspects of the present disclosure can be understood by thoseskilled in the art in light of the description, the claims, and thedrawings of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and form a partof the specification, illustrate embodiments of the present disclosureand, together with the description, further serve to explain theprinciples of the present disclosure and to enable a person skilled inthe pertinent art to make and use the present disclosure.

FIG. 1 illustrates a schematic top-down view of an exemplarythree-dimensional (3D) memory die, according to some embodiments of thepresent disclosure.

FIG. 2 illustrates a schematic top-down view of a region of 3D memorydie, according to some embodiments of the present disclosure.

FIG. 3 illustrates a perspective view of a portion of an exemplary 3Dmemory array structure, in accordance with some embodiments of thepresent disclosure.

FIG. 4 illustrates a cross-sectional view of 3D memory device withmultiple decks, according to some embodiments of the present disclosure.

FIG. 5 illustrates a schematic circuit diagram of a 3D memory devicewith multiple decks, according to some embodiments of the presentdisclosure.

FIGS. 6-8 illustrate wave form diagrams of erase operations for 3Dmemory devices with multiple decks, according to some embodiments of thepresent disclosure.

FIG. 9 illustrates a flow diagram of an erase method for a 3D memorydevice with multiple decks, according to some embodiments of the presentdisclosure.

FIGS. 10-11 illustrate wave form diagrams of erase operations for 3Dmemory devices with multiple decks, according to some embodiments of thepresent disclosure.

The features and advantages of the present invention will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings, in which like reference charactersidentify corresponding elements throughout. In the drawings, likereference numbers generally indicate identical, functionally similar,and/or structurally similar elements. The drawing in which an elementfirst appears is indicated by the leftmost digit(s) in the correspondingreference number.

Embodiments of the present disclosure will be described with referenceto the accompanying drawings.

DETAILED DESCRIPTION

Although specific configurations and arrangements are discussed, itshould be understood that this is done for illustrative purposes only. Aperson skilled in the pertinent art will recognize that otherconfigurations and arrangements can be used without departing from thespirit and scope of the present disclosure. It will be apparent to aperson skilled in the pertinent art that the present disclosure can alsobe employed in a variety of other applications.

It is noted that references in the specification to “one embodiment,”“an embodiment,” “an example embodiment,” “some embodiments,” etc.,indicate that the embodiment described can include a particular feature,structure, or characteristic, but every embodiment can not necessarilyinclude the particular feature, structure, or characteristic. Moreover,such phrases do not necessarily refer to the same embodiment. Further,when a particular feature, structure or characteristic is described inconnection with an embodiment, it would be within the knowledge of aperson skilled in the pertinent art to affect such feature, structure orcharacteristic in connection with other embodiments whether or notexplicitly described.

In general, terminology can be understood at least in part from usage incontext. For example, the term “one or more” as used herein, dependingat least in part upon context, can be used to describe any feature,structure, or characteristic in a singular sense or can be used todescribe combinations of features, structures or characteristics in aplural sense. Similarly, terms, such as “a,” “an,” or “the,” again, canbe understood to convey a singular usage or to convey a plural usage,depending at least in part upon context. In addition, the term “basedon” can be understood as not necessarily intended to convey an exclusiveset of factors and may, instead, allow for existence of additionalfactors not necessarily expressly described, again, depending at leastin part on context.

It should be readily understood that the meaning of “on,” “above,” and“over” in the present disclosure should be interpreted in the broadestmanner such that “on” not only means “directly on” something, but alsoincludes the meaning of “on” something with an intermediate feature or alayer there between. Moreover, “above” or “over” not only means “above”or “over” something, but can also include the meaning it is “above” or“over” something with no intermediate feature or layer there between(i.e., directly on something).

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper,” and the like, can be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or process step in addition to the orientation depicted inthe figures. The apparatus can be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein can likewise be interpreted accordingly.

As used herein, the term “substrate” refers to a material onto whichsubsequent material layers are added. The substrate includes a “top”surface and a “bottom” surface. The top surface of the substrate istypically where a semiconductor device is formed, and therefore thesemiconductor device is formed at a top side of the substrate unlessstated otherwise. The bottom surface is opposite to the top surface andtherefore a bottom side of the substrate is opposite to the top side ofthe substrate. The substrate itself can be patterned. Materials added ontop of the substrate can be patterned or can remain unpatterned.Furthermore, the substrate can include a wide array of semiconductormaterials, such as silicon, germanium, gallium arsenide, indiumphosphide, etc. Alternatively, the substrate can be made from anelectrically non-conductive material, such as a glass, a plastic, or asapphire wafer.

As used herein, the term “layer” refers to a material portion includinga region with a thickness. A layer has a top side and a bottom sidewhere the bottom side of the layer is relatively close to the substrateand the top side is relatively away from the substrate. A layer canextend over the entirety of an underlying or overlying structure, or canhave an extent less than the extent of an underlying or overlyingstructure. Further, a layer can be a region of a homogeneous orinhomogeneous continuous structure that has a thickness less than thethickness of the continuous structure. For example, a layer can belocated between any set of horizontal planes between, or at, a topsurface and a bottom surface of the continuous structure. A layer canextend horizontally, vertically, and/or along a tapered surface. Asubstrate can be a layer, can include one or more layers therein, and/orcan have one or more layer thereupon, there above, and/or there below. Alayer can include multiple layers. For example, an interconnect layercan include one or more conductive and contact layers (in whichcontacts, interconnect lines, and/or vertical interconnect accesses(VIAs) are formed) and one or more dielectric layers.

In the present disclosure, for ease of description, “tier” is used torefer to elements of substantially the same height along the verticaldirection. For example, a word line and the underlying gate dielectriclayer can be referred to as “a tier,” a word line and the underlyinginsulating layer can together be referred to as “a tier,” word lines ofsubstantially the same height can be referred to as “a tier of wordlines” or similar, and so on.

As used herein, the term “nominal/nominally” refers to a desired, ortarget, value of a characteristic or parameter for a component or aprocess step, set during the design phase of a product or a process,together with a range of values above and/or below the desired value.The range of values can be due to slight variations in manufacturingprocesses or tolerances. As used herein, the term “about” indicates thevalue of a given quantity that can vary based on a particular technologynode associated with the subject semiconductor device. Based on theparticular technology node, the term “about” can indicate a value of agiven quantity that varies within, for example, 10-30% of the value(e.g., ±10%, ±20%, or ±30% of the value).

In the present disclosure, the term“horizontal/horizontally/lateral/laterally” means nominally parallel toa lateral surface of a substrate, and the term “vertical” or“vertically” means nominally perpendicular to the lateral surface of asubstrate.

As used herein, the term “3D memory” refers to a three-dimensional (3D)semiconductor device with vertically oriented strings of memory celltransistors (referred to herein as “memory strings,” such as NANDstrings) on a laterally-oriented substrate so that the memory stringsextend in the vertical direction with respect to the substrate.

FIG. 1 illustrates a top-down view of an exemplary three-dimensional(3D) memory device 100, according to some embodiments of the presentdisclosure. The 3D memory device 100 can be a memory chip (package), amemory die or any portion of a memory die, and can include one or morememory planes 101, each of which can include a plurality of memoryblocks 103. Identical and concurrent operations can take place at eachmemory plane 101. The memory block 103, which can be megabytes (MB) insize, is the smallest size to carry out erase operations. Shown in FIG.1, the exemplary 3D memory device 100 includes four memory planes 101and each memory plane 101 includes six memory blocks 103. Each memoryblock 103 can include a plurality of memory cells, where each memorycell can be addressed through interconnections such as bit lines andword lines. The bit lines and word lines can be laid out perpendicularly(e.g., in rows and columns, respectively), forming an array of metallines. The direction of bit lines and word lines are labeled as “BL” and“WL” in FIG. 1. In this disclosure, memory block 103 is also referred toas a “memory array” or “array.” The memory array is the core area in amemory device, performing storage functions.

The 3D memory device 100 also includes a periphery region 105, an areasurrounding memory planes 101. The periphery region 105 contains manydigital, analog, and/or mixed-signal circuits to support functions ofthe memory array, for example, page buffers, row and column decoders andsense amplifiers. Peripheral circuits use active and/or passivesemiconductor devices, such as transistors, diodes, capacitors,resistors, etc., as would be apparent to a person of ordinary skill inthe art.

It is noted that, the arrangement of the memory planes 101 in the 3Dmemory device 100 and the arrangement of the memory blocks 103 in eachmemory plane 101 illustrated in FIG. 1 are only used as an example,which does not limit the scope of the present disclosure.

Referring to FIG. 2, an enlarged top-down view of a region 108 in FIG. 1is illustrated, according to some embodiments of the present disclosure.The region 108 of the 3D memory device 100 can include a staircaseregion 210 and a channel structure region 211. The channel structureregion 211 can include an array of memory strings 212, each including aplurality of stacked memory cells. The staircase region 210 can includea staircase structure and an array of contact structures 214 formed onthe staircase structure. In some embodiments, a plurality of slitstructures 216, extending in WL direction across the channel structureregion 211 and the staircase region 210, can divide a memory block intomultiple memory fingers 218. At least some slit structures 216 canfunction as the common source contact (e.g., array common source) for anarray of memory strings 212 in channel structure regions 211. A topselect gate cut 220 can be disposed, for example, in the middle of eachmemory finger 218 to divide a top select gate (TSG) of the memory finger218 into two portions, and thereby can divide a memory finger into twomemory slices 224, where memory cells in a memory slice 224 that sharethe same word line form a programmable (read/write) memory page. Whileerase operation of a 3D NAND memory can be carried out at memory blocklevel, read and write operations can be carried out at memory pagelevel. A memory page can be kilobytes (KB) in size. In some embodiments,region 108 also includes dummy memory strings 222 for process variationcontrol during fabrication and/or for additional mechanical support.

FIG. 3 illustrates a perspective view of a portion of an exemplarythree-dimensional (3D) memory array structure 300, according to someembodiments of the present disclosure. The memory array structure 300includes a substrate 330, an insulating film 331 over the substrate 330,a tier of lower select gates (LSGs) 332 over the insulating film 331,and a plurality of tiers of control gates 333, also referred to as “wordlines (WLs),” stacking on top of the LSGs 332 to form a film stack 335of alternating conductive and dielectric layers. The dielectric layersadjacent to the tiers of control gates are not shown in FIG. 3 forclarity.

The control gates of each tier are separated by slit structures 216-1and 216-2 through the film stack 335. The memory array structure 300also includes a tier of top select gates (TSGs) 334 over the stack ofcontrol gates 333. The stack of TSG 334, control gates 333 and LSG 332is also referred to as “gate electrodes.” The memory array structure 300further includes memory strings 212 and doped source line regions 344 inportions of substrate 330 between adjacent LSGs 332. Each memory strings212 includes a channel hole 336 extending through the insulating film331 and the film stack 335 of alternating conductive and dielectriclayers. Memory strings 212 also includes a memory film 337 on a sidewallof the channel hole 336, a channel layer 338 over the memory film 337,and a core filling film 339 surrounded by the channel layer 338. Amemory cell 340 can be formed at the intersection of the control gate333 and the memory string 212. A portion of the channel layer 338underneath the control gate 333 is also referred to as the channel ofthe memory cell 340. The memory array structure 300 further includes aplurality of bit lines (BLs) 341 connected with the memory strings 212over the TSGs 334. The memory array structure 300 also includes aplurality of metal interconnect lines 343 connected with the gateelectrodes through a plurality of contact structures 214. The edge ofthe film stack 335 is configured in a shape of staircase to allow anelectrical connection to each tier of the gate electrodes.

In FIG. 3, for illustrative purposes, three tiers of control gates333-1, 333-2, and 333-3 are shown together with one tier of TSG 334 andone tier of LSG 332. In this example, each memory string 212 can includethree memory cells 340-1, 340-2 and 340-3, corresponding to the controlgates 333-1, 333-2 and 333-3, respectively. In some embodiments, thenumber of control gates and the number of memory cells can be more thanthree to increase storage capacity. The memory array structure 300 canalso include other structures, for example, TSG cut, common sourcecontact (i.e., array common source) and dummy memory string. Thesestructures are not shown in FIG. 3 for simplicity.

FIG. 4 illustrates a cross-sectional view (along X direction) of a 3Dmemory device 400 with multiple decks, according to some embodiments ofthe present disclosure. As an example in FIG. 4, the 3D memory device400 has two decks, i.e., a bottom deck 450 and a top deck 452 stackedvertically on top of the bottom deck 450. The bottom deck 450 and thetop deck 452 are disposed on a substrate (e.g., the substrate 330).Substrate 330 can provide a platform for the 3D memory device 400, whichis formed on a front (e.g., top) surface 330 f of the substrate 330. Andsubsequent structures are formed in a vertical direction (e.g.,orthogonal to the front surface of substrate 330). In FIG. 4, the X andY directions are along a plane parallel to the front and back surfacesof substrate 330, while the Z direction is in a direction orthogonal tothe front and back surfaces of substrate 330. Here, the X and Ydirections are parallel to the respective word line (WL) and bit line(BL) directions shown in FIGS. 1-3.

In some embodiments, the substrate 330 can provide a platform forforming subsequent structures. In some embodiments, the substrate 330can be any suitable semiconductor substrate having any suitablesemiconductor materials, such as monocrystalline, polycrystalline orsingle crystalline semiconductors. For example, the substrate 330 caninclude silicon, silicon germanium (SiGe), germanium (Ge), silicon oninsulator (SOI), germanium on insulator (GOI), gallium arsenide (GaAs),gallium nitride, silicon carbide, III-V compound, or any combinationsthereof. In some embodiments, the substrate 330 can include a layer ofsemiconductor material formed on a handle wafer, for example, glass,plastic, or another semiconductor substrate.

A front surface 330 f of the substrate 330 is also referred to as a“main surface” or a “top surface” of the substrate herein. Layers ofmaterials can be disposed on the front surface 330 f of the substrate330. A “topmost” or “upper” layer is a layer farthest or farther awayfrom the front surface 330 f of the substrate. A “bottommost” or “lower”layer is a layer closest or closer to the front surface 330 f of thesubstrate.

In some embodiments, the bottom deck 450 and the top deck 452 includebottom film stack 335-1 and top film stack 335-2, respectively. Thebottom film stack 335-1 and the top film stack 335-2 can be similar tothe film stack 335 in FIG. 3, and include a plurality of conductorlayers 454 and dielectric layers 456. The conductor layers 454 and thedielectric layers 456 in the bottom film stack 335-1 and top film stack335-2 alternate in the vertical direction. In other words, except theone at the bottom of the bottom film stack 335-1 and the one at the topof the top film stack 335-2, each conductor layer 454 can be sandwichedby two dielectric layers 456 on both sides, and each dielectric layer456 can be sandwiched by two conductor layers 454 on both sides. Theconductor layers 454 can each have the same thickness or have differentthicknesses. Similarly, the dielectric layers 456 can each have the samethickness or have different thicknesses. In some embodiments, the bottomfilm stack 335-1 and the top film stack 335-2 include different numberof conductor layers 454 and/or dielectric layers 456 and/or includedifferent materials and/or thicknesses. The conductor layers 454 caninclude conductor materials such as W, Co, Cu, Al, Ti, Ta, TiN, TaN, Ni,doped silicon, silicides (e.g., NiSix, WSix, CoSix, TiSix) or anycombination thereof. The dielectric layers 456 can include dielectricmaterials such as silicon oxide, silicon nitride, silicon oxynitride, orany combination thereof.

In some embodiments, the bottom deck 450 and the top deck 452 can alsoinclude memory cells of a 3D NAND memory (e.g., the memory cell 340 inFIG. 3) where the memory cells 340 can be stacked vertically as memorystrings (e.g, the memory strings 212 in FIG. 3). As shown in FIG. 4, thememory string 212 extends through the top film stack 335-2 and thebottom film stack 335-1 and includes a bottom vertical structure 212-1and a top vertical structure 212-2. Each memory string 212 can includethe channel layer 338 and the memory film 337 (similar to those in FIG.3). In some embodiments, the channel layer 338 includes silicon, such asamorphous silicon, polysilicon, or single crystalline silicon. In someembodiments, the memory film 337 is a composite layer including atunneling layer, a storage layer (also known as “charge trap/storagelayer”), and a blocking layer. Each memory string 212 can have acylinder shape (e.g., a pillar shape). The channel layer 338, thetunneling layer, the storage layer, and the blocking layer are arrangedalong a direction from the center toward the outer surface of the pillarin this order, according to some embodiments. The tunneling layer caninclude silicon oxide, silicon nitride, or any combination thereof. Theblocking layer can include silicon oxide, silicon nitride, highdielectric constant (high-k) dielectrics, or any combination thereof.The storage layer can include silicon nitride, silicon oxynitride,silicon, or any combination thereof. In some embodiments, the memoryfilm 337 includes ONO dielectrics (e.g., a tunneling layer includingsilicon oxide, a storage layer including silicon nitride, and a blockinglayer including silicon oxide).

In some embodiments, the conductor layers 454 in the bottom film stack335-1 and the top film stack 335-2 can act as the control gates (e.g.,the control gates 333 in FIG. 3) for the memory cells 340. As shown inFIG. 4, the memory string 212 can include the lower select gate 332(e.g., a source select gate) at a lower end (i.e., a source terminal) ofthe memory string 212 in the bottom deck 450. The memory string 212 canalso include the top select gate 334 (e.g., a drain select gate) at anupper end (i.e. a drain terminal) of the memory string 212 in the topdeck 452. As used herein, the “upper end” of a component (e.g., memorystring 212) is the end further away from the substrate 330 in thevertical direction, and the “lower end” of the component (e.g., memorystring 212) is the end closer to the substrate 330 in the verticaldirection. As shown in FIG. 4, for each memory string 212, the drainselect gate 334 can be above the source select gate 332. In someembodiments, the lower select gate 332 and top select gate 334 includeconductor materials such as W, Co, Cu, Al, doped silicon, silicides, orany combination thereof.

In some embodiments, the 3D memory device 400 includes an epitaxiallayer 458 on an lower end of the channel layer 338 of the memory string212 in the bottom deck 450. The epitaxial layer 458 can include asemiconductor material, such as silicon. The epitaxial layer 458 can beepitaxially grown from the substrate 330. For each memory string 212,the epitaxial layer 458 is referred to herein as an “epitaxial plug.”The epitaxial plug 458 can contact both the channel layer 338 and anarray cell well-body 345 in the substrate 330. The epitaxial plug 458can function as the channel of the lower selective gate 332 at the lowerend of memory string 212. In some embodiments, the array cell well-body345 can be p-type doped, while the doped source line region 344 can ben-type doped.

In some embodiments, the 3D memory device 400 includes an insulatinglayer 462 in both the top deck 452 and the bottom deck 450. Theinsulating layer 462 surrounds the memory strings 212 and provideisolation between neighboring memory strings 212. The insulating layer462 can include insulating materials, for example, silicon oxide,silicon nitride, silicon oxynitride, doped silicon oxide (such as F-,C-, N- or H-doped oxides), tetraethoxysilane (TEOS), polyimide,spin-on-glass (SOG), low-k dielectric material such as porous SiCOH,silsesquioxan (SSQ), or any combination thereof.

In some embodiments, the top vertical structure 212-2 of the memorystring 212 in the top deck 452 can be substantially aligned with thebottom vertical structure 212-1 in the bottom deck 450. In someembodiments, each memory string 212 includes a conductive plug 460 inbetween the top vertical structure 212-2 and the bottom verticalstructure 212-1. The conductive plug 460 can provide electric connectionbetween the channel layer 338 located in the bottom vertical structure212-1 and the channel layer 338 located in the top vertical structure212-2 such that the top vertical structure 212-2 and the bottom verticalstructure 212-1 can form the memory string 212 for the 3D memory device400 of two decks, where the memory string 212 can provide similar memoryfunctions as the memory string 212 in a 3D memory device with a singledeck. In some embodiments, the conductive plug 460 includes polysilicon.

According to some embodiments, the 3D memory device 400 further includesa conductive recess 461 on a top portion of the memory string 212 in thetop deck 452. In some embodiments, the conductive recess 461 includespolysilicon.

In some embodiments, the film stack 335-1 and film stack 335-2 eachincludes a staircase structure where each of the conductive layer 454terminate at a different length in the horizontal ‘X’ direction. Thestaircase structure allows for electric connection between the word lineand the conductive layer 454.

In some embodiments, the 3D memory device 400 includes an array commonsource (ACS) 464 penetrating vertically through the top deck 452 and thebottom deck 450 into the substrate 330. The ACS 464 can be electricallyconnected to the doped source line region 344. Accordingly, the ACS 464can be electrically connected to the lower end (i.e., the sourceterminal) of the memory string 212 if the lower select gate 332 isswitched on.

In some embodiments, the ACS 464 includes a conductive core that can bea metal or metal alloy such as tungsten (W), aluminum (Al), titanium(Ti), copper (Cu), cobalt (Co), nickel (Ni), titanium nitride (TiN),tungsten nitride (WN), tantalum (Ta), tantalum nitride (TaN), AlTi, orany combination thereof. The arrangement of the array common source 464in FIG. 4 is shown for illustration purpose. In some embodiments, theACS 464 can be extended along the X-direction (parallel to word linesshown in FIGS. 1-3). In some embodiments, the ACS 464 can be formed on aback side of the substrate 330, opposite to the front surface.

The 3D memory device 400 can also include bit line contacts (not shownin FIG. 4) formed on a top end (i.e., a drain terminal) of the memorystrings 212 in the top deck 452 to provide individual access to thechannel layers 338 of the memory strings 212 in the top deck 452 andbottom deck 450. The conductive lines connected with the conductivelayers 454 and the bit line contacts form word lines and bit lines ofthe 3D memory device 400, respectively. Typically the word lines and bitlines are laid perpendicular to each other (e.g., in rows and columns,respectively), forming an “array” of the memory.

It is noted that a 3D memory device with only two decks are describedhere for illustration purpose. Similar structures and functions can beextended to a 3D memory device with more than two decks.

FIG. 5 shows a schematic circuit diagram of a 3D memory device 500 withmultiple decks, according to some embodiments of the present disclosure.As an example, the 3D memory device 500 includes two decks, e.g., thetop deck 452 and the bottom deck 450. The 3D memory device 500 alsoincludes the memory string 212 having a plurality of stacked memorycells 340, where the memory cells 340-1 are in the bottom deck 450 andthe memory cells 340-2 are in the top deck 452. The 3D memory device 500also includes a conductive plug 460 in between the top deck 452 and thebottom deck 450. Accordingly, in the 3D memory device 500 with multipledecks, the memory cells 340-2 in the top deck 452 can be electricallyconnected with the memory cells 340-1 in the bottom deck 450 to form thememory string 212. The memory string 212 further includes at least onefield effect transistor (e.g., MOSFET) at each end, which is controlledby the lower select gate (LSG) 332 and the top select gate (TSG) 334,respectively. And the two respective transistors are referred to as alower select transistor 332-T and a top select transistor 334-T. Thestacked memory cells 340 can be controlled by the control gates 333,where the control gates 333 are connected to word lines (not shown) ofthe 3D memory device 500. The drain terminal of the top selecttransistor 334-T can be connected to the bit line 341, and the sourceterminal of the lower select transistor 332-T can be connected to thedoped source line region 344 (see FIG. 4), from where the ACS 464 can beformed. The ACS 464 can be shared by the memory strings 212 in an entirememory block, and is also referred to as the common source line. Asshown in FIG. 5, the top deck 452 is coupled to the ACS 464 (i.e., acommon source line in the substrate) through the bottom deck 450.

In a NAND memory, read and write operations can be performed in a memorypage, which includes memory cells sharing the same word line. Eachmemory cell can be in an erased state or a programmed state. Initially,all memory cells in a block can be reset to the erased state as logic“1” by implementing a negative voltage difference between the controlgates 333 and source terminals of the memory cells (e.g., lower terminalcloser to the substrate 330) such that all electronic charges stored inthe memory cells can be removed. In some embodiments, electrons storedin the memory film can be expelled to the channel layer throughFlower-Nordheim (F-N) tunneling, and thereby changing a thresholdvoltage V_(th) of the memory cell 340. In some embodiments, the ACS 464can be grounded at zero voltage (0 V) and a negative voltage can beapplied on the control gates 333. In some embodiments, a positivevoltage can be applied at the ACS 464 and the control gates 333 can begrounded at 0 V. At the erased state, the threshold voltage V_(th) ofthe memory cells 340 can be reset to a lowest value, e.g., an erasedstate threshold voltage V_(th_erase), and the corresponding currentflowing through the memory cells 340 between the source terminal (e.g.,terminal closer to the substrate 330) and the drain terminal (e.g.,terminal closer to the bit lines 341) can be at the highest level.

FIG. 6 shows a wave form for an erase operation 600, according to someembodiments of the present disclosure. The erase operation 600 can beperformed for all the memory cells in a memory block (e.g., the memoryblock 103 in FIG. 1). For the 3D memory devices with multiple decksshown in FIGS. 4 and 5, all the memory cells in the top deck 452 andbottom deck 450 can be erased simultaneously. For example, during a timeperiod t_(erase), all the memory cells 340 of the memory string 212 inthe 3D memory device 500 (in FIG. 5), including in the top deck 452 andin the bottom deck 450, can be reset to the erased state. The eraseoperation 600 includes applying an erase voltage V_(erase) at the ACS464 and/or the bit line 341 at to. Simultaneously, a hold-releasevoltage V_(hld-re) can be applied to the control gates 333 (includingthe control gates 333-1 in the bottom deck 450 and the control gates333-2 in the top deck 452). In some embodiments, the erase voltageV_(erase) can be in a range between about 15 V to about 25V. In someembodiments, the erase voltage V_(erase) can be about 20 V. Whenapplying the hold-release voltage V_(hld-re) on an electrode, a voltageof 0 V is applied followed by release of the voltage of 0 V. In theother words, the electrode applied with the hold-release voltageV_(hld-re), is biased at the voltage of 0 V first (e.g. at t₀) and thenthe external bias is removed and the electrode is released to floating(e.g., at t₁, where t₁>t₀) where potential of the electrode can bechanged by electromagnetic fields or charge accumulation within theconductor.

After initially applying the hold-release voltage V_(hld-re), unselectedword lines (i.e., control gates of erase-inhibited memory cells) can bekept floating after t₁, where potential of the unselected word lines canrise to a voltage V_(H) at t₃ (t₃>t₂>t₁>t₀) through capacitive coupling.In some embodiments, the voltage V_(H) can have a magnitude close to theerase voltage V_(erase).

For selected word lines (i.e., control gates of memory cells to beerased), a low voltage V_(L) can be applied at t₂ (t₂>t₁>t₀), wherepotential of the selected word lines can reach the low voltage V_(L) att₃. In some embodiments, the low voltage V_(L) can be any voltage lessthan about 1 V. In some embodiments, the low voltage V_(L) can be 0 V.In this example, a voltage difference between the selected word line andthe ACS 464 is V_(L)-V_(erase), which can be high enough to remove thestored charges in the memory cells and reset the memory cells to theerased state.

In some embodiments, during the erase operation, the bit lines 341 andthe top select gates (TSGs) 334 can be floating, i.e., not applied withany voltage. In this example, the bit line 341 and the top selecttransistor 334-T can be coupled to a high electric potential due toparasitic capacitances between the bit line 341 and the ACS 464. In themeantime, after applying the erase voltage V_(erase) on the ACS 464, theelectric potential of the channel of the memory cells 340 increasesaccordingly. Charge carriers (e.g., holes) in the doped source lineregion 344 (see FIG. 4) can be injected into the channel layer 338.Because the mobility of holes in the channel layer can be low in thechannel layer 338 made of polycrystalline silicon or amorphous silicon,the electric potential of the channel of the memory cell 340 risesgradually from the bottom to the top of the channel. Accordingly, theelectric potential of the channel layer 338 in the memory string 212rises from bottom (i.e. the doped source line region 344 in FIG. 4 orthe ACS 464 in FIG. 5) to the top (close to the bit line 341) gradually.

In some embodiments, after the erase operation, a distribution of thethreshold voltages V_(th) of the memory cells 340 can be verified,during an erase verification operation, by measuring the current flowingthrough the memory cells 340. For example, a verifying voltage V₁ (e.g.,0 V) can be applied to the control gates 333. For the memory cell withthe threshold voltage V_(th) lower than the verifying voltage V₁, thecorresponding memory cell can be switched on and form a conductive path.If the threshold voltage V_(th) of the memory cell is higher than theverifying voltage V₁, the corresponding memory cell is switched off. Ifnot all the memory cells have threshold voltages less than the verifyingvoltage V₁, a higher voltage can then be applied by adding a stepvoltage V_(step) to again verify the erased state. The aboveverification steps can be repeated until the threshold voltages of allthe memory cells are verified. For the memory cells having the thresholdvoltages higher than a target erase-verify voltage V_(target) after theerase operation, the memory cells are considered as erase fail. In someembodiments, the distribution of the threshold voltages V_(th) of thememory cells 340 can be in a range between about −4.0 V to about −2.0 Vafter an erase operation, and the target erase-verify voltage V_(target)can be in a range between about −2.0V to about −1.0V.

In a NAND flash memory, the memory cells can be reset to the erasedstates in an entire memory block which can be as large as hundreds ofkilobytes (KB) or even megabytes (MB) in size, and the erase operationcan take hundreds of microseconds to complete, much longer than a randomaccess time for read or write. Thus, it is desirable to reduce thememory block size for a faster erase operation. Without scarifyingstorage capacity, the erase operation can be performed based on asub-block, for example, for a selected deck in a 3D memory device withmultiple decks.

FIG. 7 shows a wave form for an erase operation 700, according to someembodiments of the present disclosure. In this example, for the 3Dmemory device 500 in FIG. 5, the erase operation 700 can be performedduring the time period t_(erase) for the memory cells 340-1 in thebottom deck 450, while the memory cells 340-2 in the top deck 452 can beinhibited from being erased, i.e., the data stored in the memory cells340-2 in the top deck 452 remain after the erase operation. In thisexample, selected word lines, e.g., the control gates 333-1 of thebottom deck 450 can be applied with the hold-release voltage V_(hld-re)at t₀/t₁, and the low voltage V_(L) at t₂ (t₂>t₁>t₀). Similar to theerase operation 600, the ACS 464 and/or the bit lines 341 can be appliedwith the erase voltage V_(erase). As a result, a negative voltagedifference can be established between the control gates 333-1 in thebottom deck 450 and the source terminals of the memory cells 340-1 inthe bottom deck 450. At the end of the time period t_(erase), the memorycells 340-1 in the bottom deck 450 can be reset to the erased state. Inthis example, unselected word lines, e.g., the control gates 333-2 inthe top deck 452, can be applied with the hold-release voltageV_(hld-re) at t₀/t₁ and kept floating after t₁ (i.e., not applied withany external voltage). Potential of the control gates 333-2 in the topdeck 452 can rise to the voltage V_(H) through capacitive coupling.

FIG. 8 shows a wave form for an erase operation 800, according to someembodiments of the present disclosure. In this example, the eraseoperation 800 can be performed for the memory cells 340-2 in the topdeck 452, where the selected word lines, e.g., the control gates 333-2in the top deck 452 can be applied with the hold-release voltageV_(hld-re) at t₀/t₁, and the low voltage V_(L) at t₂. Similar to theerase operation 600, the ACS 464 and/or the bit lines 341 can be appliedwith the erase voltage V_(erase). As a result, a negative voltagedifference can be established between the control gates 333-2 in the topdeck 452 and the source terminals of the memory cells 340-2 in the topdeck 452. At the end of the time period t_(erase), the memory cells340-2 in the top deck 452 can be reset to the erased state. In thisexample, unselected word lines, e.g., the control gates 333-1 in thebottom deck 450 can be applied with the hold-release voltage V_(hld-re)at t₀/t₁ and kept floating after t₁, i.e., not applied with any externalvoltage. Potential of the control gates 333-1 in the bottom deck 450 canrise to the voltage V_(H) through capacitive coupling. In someembodiments, the voltage V_(H) can have a magnitude close to the erasevoltage V_(erase), where the voltage difference between the control gate333-1 and the source terminals of the memory cells 340-1 can be verysmall. Accordingly, the memory cells 340-1 in the bottom deck 450 can beinhibited from being erased, i.e., the data stored in the memory cells340-1 in the bottom deck 450 remain after the erase operation.

However, in the example shown in FIG. 8, the states and data pattern ofthe memory cells 340-1 in the bottom deck 450 during the erase operation800 can impact the erased state threshold voltages V_(th_erase) of thememory cells 340-1 in the top deck 452. As discussed previously, afterthe erase voltage V_(erase) is applied on the ACS 464, the chargecarriers (e.g., holes) are injected to the channels of the memory cells340 from bottom to the top, and electric potential of the channels risesgradually from bottom to top. However, the state or data pattern of thememory cells 340-1 in the bottom deck affect the electric potential ofthe corresponding memory cells 340-1, and thereby affect the transportof the charge carriers from the ACS 464 (or the doped source line region344 in FIG. 4) to the bit line 341. As a result, the electric potentialat the conductive plug 460 in FIG. 5 can be lower than the erase voltageV_(erase) applied on the ACS 464. When the electric potential of thememory cells 340-2 in the top deck 452 is significantly lowered from theerase voltage V_(erase), the voltage difference between the controlgates 333-2 and the channel of the memory cells 340-2 may not be highenough to reset the correspond memory cell 340-2 to the erased statewith desirable threshold voltage, i.e., the erased state thresholdvoltage V_(th_erase) below the target erase-verify voltage V_(target).When the erased state threshold voltages V_(th_erase) of the memorycells 340-1 in the top deck 452 shift to a higher value, more memorycells having the erased state threshold voltages V_(th_erase) higherthan the target erase-verify voltage V_(target) and thereby more memorycells have erase fail.

Therefore, in some embodiments of the present disclosure, beforeperforming an erase operation, preparation steps are added. During thepreparation steps, the state and/or data pattern of neighboring deckscan be checked and the memory cells of the neighboring decks can beaddressed accordingly to facilitate the charge carriers transport fromthe ACS 464 (or the doped source line region 344 in FIG. 4) to the bitline 341 and thereby reduce the number of erase fails.

FIG. 9 illustrates a flow diagram of an erase method 900 for the 3Dmemory device 500 in FIG. 5, according to some embodiments of thepresent disclosure. It should be understood that the erase method 900are not exhaustive and that other operation steps can be performed aswell before, after, or between any of the illustrated operation steps.In some embodiments, some operation steps of exemplary erase method 900can be omitted or other operation steps can be included, which are notdescribed here for simplicity. In some embodiments, operation steps oferase method 900 can be performed in a different order and/or vary.

According to some embodiments of this disclosure, the erase method 900can be implemented for the 3D memory device 500 with the top deck 452and the bottom deck 450 shown in FIG. 5. However, the method providedhere can be extended to any 3D memory device with multiple decks stackedvertically, where the erase method can be performed for a selected deck.The bias conditions are similar to the ones discussed previously throughFIGS. 6-8, where the differences are discussed below.

In the example of erase method 900, the erase voltage V_(erase) isapplied at the ACS 464. In some embodiments, the erase voltage V_(erase)can be applied at the bit line 341 and the discussion below regarding tothe top and bottom decks can be then applied to the bottom and topdecks, respectively. In some embodiments, the erase voltage V_(erase)can be applied at the ACS 464 and the bit line 341, where the discussionbelow regarding to the top and bottom decks can be extended to anyselected deck and unselected deck.

The erase method 900 for the 3D memory device 500 starts at operationstep S910.

At operation step S915, it is evaluated if a target memory deck to beerased is the bottom deck 450.

If the memory cells 340-1 of the bottom deck 450 are to be erased, thenat operation step S920, the erase voltage V_(erase) can be applied tothe array common source (ACS) 464 at to and the hold-release voltageV_(hld-re) can be applied to the control gates 333 at t₀/t₁ (t₁>t₀),where 0 V can be applied at to and then removed at t₁. The control gates333-2 of the top deck 452 (i.e., unselected word lines in this example)can be kept floating after t₁ (i.e., without applying any externalvoltages). The control gates 333-1 of the bottom deck 450 (i.e.,selected word lines in this example) can be applied with the low voltageV_(L) at t₂ (t₂>t₁>t₀). The details have been discussed previously alongwith FIG. 7. Because the target memory deck is the bottom deck 450,close to the ACS 464, the charge carriers can be injected from the ACS464 (or the doped source line region 344 in FIG. 4) to the channel ofthe memory cells 340-1 without being affected by the states or datapattern from the top deck 452. Therefore, the electric channel potentialof the memory cells 340-1 can rise following the erase voltage V_(erase)at the ACS 464 without being affected by the states or data pattern fromthe top deck 452. As a result, the erased state of the memory cells340-1 in the bottom deck 450 can achieve a target erase fail rate. Inthis example, the bottom deck 450 can be a selected memory block to beerased and the top deck 452 can be an unselected memory block inhibitedfrom being erased.

If at operation step S915, it is determined that the target memory deckto be erased is not the bottom deck 450, but the top deck 452, then thetop deck 452 can be a selected memory block to be erased and the bottomdeck 450 can be an unselected memory block inhibited from being erased.

At operation step S925, states and/or data pattern of the memory cells340-1 in the bottom deck 450 are checked and determined.

In some embodiments, if the bottom deck 450 is fully programed, i.e.,all the memory cells 340-1 in the bottom deck 450 are at programmedstates, then operation step S930 will be performed. In some embodiments,the programmed states include logic state “0” with a higher thresholdvoltage (e.g., a programmed threshold voltage V_(th_program)) than theerased state threshold voltage V_(th_erase). In some embodiments, usingmulti-level-cell or triple-level-cell technology, each memory cell 340-1can have multiple programed states, i.e., multiple values of thresholdvoltages V_(th). In the other words, each memory cell 340-1 can storemultiple bits, leading to significant increase of storage capacity.

At operation step S930, a first preparation voltage V_(prep1) can beapplied at the control gates 333-1 of the memory cells 340-1 in thebottom deck 450, while the ACS 464 can be grounded. The wave form ofoperation step S930 is shown in FIG. 10. In some embodiments, the firstpreparation voltage V_(prep1) can be selected such that the memory cells340-1 can be switched to accumulation mode, where charge carriers (e.g.,holes) can be accumulated in the channel. To avoid disturb the storeddata in the memory cells 340-1, the first preparation voltage V_(prep1)can be kept low. In some embodiments, the first preparation voltageV_(prep1) can be in a range between about 0 V to about 1.0 V. In someembodiments, the first preparation voltage V_(prep1) can be 0 V.

Next, at operation step S950, the first preparation voltage V_(prep1)applied on the control gates 333-1 of the memory cells 340-1 in thebottom deck 450 can be removed and the control gates 333-1 can beapplied with the hold-release voltage V_(hld-re) first and then keptfloating, i.e., without external voltage applied. Accordingly, thememory cells 340-1 can be inhibited from being erased similar to theprevious discussion with reference to FIG. 8. In the meantime, the lowvoltage V_(L) can be applied to the control gates 333-2 of the top deck452 after the hold-release voltage V_(hld-re). Because charge carriers(e.g., holes) are accumulated in the channels of the memory cells 340-1in the bottom deck 450, the charge carriers (e.g., holes) injected fromthe ACS 464 (or the doped source line region 344 in FIG. 4) to thememory cells 340-1 can be easily transported upwards. Thus, the electricpotential of the channel of the memory cells 340-1 can rise to the erasevoltage V_(erase) applied on the ACS 464. Accordingly, the electricpotential at the conductive plug 460 can be set close to the erasevoltage V_(erase), which enables the erase operation to be performed onthe memory cells 340-2 in the top deck 452, effectively not depending onthe states or data pattern of the bottom deck 450. As a result, theerased state of the memory cells 340-2 in the top deck 452 can achieve atarget erase fail rate.

If at operation step S925, it is determined that the bottom deck is notfully programmed (not all the memory cells 340-1 are at the programmedstates), it can be further checked at operation step S935 whether thebottom deck is fully erased, i.e. whether all the memory cells 340-1 areat the erased states.

If it is determined at operation step S935 that all the memory cells340-1 in the bottom deck 450 are at the erased states, operation stepS940 can be performed where a second preparation voltage V_(prep2) canbe applied at the control gates 333-1 of the memory cells 340-1 in thebottom deck 450. The wave form of the operation step S940 is shown inFIG. 11. In some embodiments, the second preparation voltage V_(prep2)can be higher than the first preparation voltage V_(prep1). In someembodiments, the second preparation voltage V_(prep2) can be in a rangebetween about 1.0 V to about 7.0 V. In some embodiments, the secondpreparation voltage V_(prep2) can be about 1.0 V. The erase method 900can then be continued following operation step S950 describedpreviously. In some embodiments, the second preparation voltageV_(prep2) can be high enough to write dummy data patterns into thememory cells 340-1, i.e., set the memory cells 340-1 to the programmedstates. In some embodiments, the dummy data patterns can be programmedto all the memory cells 340-1 in the bottom deck. In some embodiments,every the other memory cells 340-1 in the bottom deck can be programmedwith the dummy data patterns by applying the second preparation voltageV_(prep2) to the corresponding control gates 333-1. In some embodiments,the dummy data patterns can be programmed to a subset of memory cells340-1 in the bottom deck.

If at the operations step S935, it is determined that the bottom deck450 is not fully erased, the bottom deck 450 is partially programmed andpartially erased. In this example, some memory cells 340-1 are at theerased states and some memory cells 340-1 are at the programmed states.At operation step S955, the first preparation voltage V_(prep1) can beapplied to those memory cells 340-1 that are at the programmed statesand the second preparation voltage V_(prep2) can be applied to thosememory cells 340-1 that are at erased states at operation step S960. Theerase method 900 can then be continued following operation step S950described previously.

In some embodiments, the operation step S955 and operation step S960 areperformed when two neighboring memory cells 340-1 in the bottom decksare both at the erased states.

In case that data pattern recognition in the bottom deck 450 causesignificant delay, in some embodiments, the first preparation voltageV_(prep1) and the second preparation voltage V_(prep2) can be randomlyapplied to the control gates 333-1 in the bottom deck 450.

By preparing the memory cells 340-1 in the bottom deck 450 according tothe states and data pattern, the electric potential at the conductiveplug 460 can rise close to the erase voltage V_(erase) applied at theACS 464. Therefore, the memory cells 340-2 in the top deck 452 can beerased similarly to the memory cells in the bottom deck 450 or a 3Dmemory device with a single deck and the erase fail rate in the top deck452 can be reduced.

The methods described herein can also be implemented in a 3D memorydevice with multiple decks (more than two decks), where middle decks andtop decks can have similar erase fails as discussed above. By addingpreparation operations, the electric potentials at a source end of themiddle deck or top deck (e.g., conductive plugs 460) can be controlledto the erase voltage V_(erase) applied at the array common source in thesubstrate. For example, when a top deck is vertically stacked on amiddle deck and the middle deck is vertically stacked on a bottom deckover a substrate, the top, middle and bottom decks can be erasedselectively. To improve erase yield for the middle deck, the operationsteps S940, S955 and/or S960 can be implemented on the top and bottomdecks before the erasing the middle deck. In this example, at operationstep S950, the hold-release voltage can be applied on unselected wordlines of the top deck and the bottom deck, and the low voltage V_(L) canbe applied on selected word lines of the middle deck.

Similarly, the methods described herein can also be implemented in a 3Dmemory device with multiple decks, where the erase voltage V_(erase) canbe applied at the ACS 464 and/or the bit line 341. In this example, thebottom deck, middle deck or top deck can have erase fails depending onthe distance between the corresponding deck and the ACS 464 and/or thebit line 341. By adding similar preparation operations discussed above,the electric potentials at a source and/or drain end of thecorresponding deck can be controlled to the erase voltage V_(erase).Thus, sufficient voltage difference can be maintained between selectedword lines (i.e., control gates of target memory cells) and channels ofthe target memory cells. The corresponding deck (i.e., selectedsub-block) can therefore be erased successfully.

In summary, the present disclosure provides erasing methods for athree-dimensional (3D) memory device. The 3D memory device includesmultiple decks vertically stacked over a substrate, wherein each deckincludes a plurality of memory cells. The erasing method includeschecking states of the plurality of memory cells of an erase-inhibitdeck and preparing the erase-inhibit deck according to the states of theplurality of memory cells. The erasing method also includes applying anerase voltage at an array common source, applying a hold-release voltageon unselected word lines of the erase-inhibit deck, and applying a lowvoltage on selected word lines of a target deck.

The present disclosure also provides a three-dimensional (3D) memorydevice having multiple decks vertically stacked over a substrate. Eachdeck includes a film stack of alternating conductive and dielectriclayers, and a plurality of memory strings penetrating through the filmstack, where each memory string includes a plurality of memory cells.The 3D memory device also includes a conductive plug disposed betweenadjacent decks, electrically connecting the plurality of memory stringsbetween the adjacent decks. The multiple decks of the 3D memory deviceincludes a target deck and an erase-inhibit deck. The 3D memory deviceis configured to erase the target deck by the following steps: checkingstates of the plurality of memory cells of the erase-inhibit deck,preparing the erase-inhibit deck according to the states of theplurality of memory cells of the erase-inhibit deck, applying an erasevoltage at an array common source, applying a hold-release voltage onunselected word lines of the erase-inhibit deck, and applying a lowvoltage on selected word lines of the target deck.

The present disclosure further provides a method for erasing a selectedmemory block of a three-dimensional (3D) memory device, wherein theselected memory block is stacked on an unselected memory block on asubstrate and the selected memory block is coupled to a common sourceline in the substrate through the unselected memory block. The methodincludes applying, to a first set of word lines of the unselected memoryblock, a preparation voltage based on states of memory cells in theunselected memory block; floating the first set of word lines to inhibiterasing the unselected memory block; applying an erase voltage to thecommon source line; and applying a low voltage to a second set of wordlines of the selected memory block to erase the selected memory block.

The present disclosure also provides an erasing method for athree-dimensional (3D) memory device having a plurality of memory cellsvertically stacked on a substrate. The erasing method includes applyinga first preparation voltage to a first word line corresponding to afirst memory cell, wherein the first memory cell is at a programmedstate; and applying a second preparation voltage to a second word linecorresponding to a second memory cell, wherein the second memory cell isat an erased state. The erasing method also includes floating the firstword line and the second word line to inhibit erasing the first memorycell and the second memory cell; applying a low voltage to a third wordline of a third memory cell to erase the third memory cell, wherein thethird memory cell is stacked on the first memory cell and the secondmemory cell; and applying an erase voltage to a common source lineshared by the first memory cell, the second memory cell and the thirdmemory cell.

The present disclosure further provides a three-dimensional (3D) memorydevice including a first memory block disposed on a substrate and asecond memory block stacked on the first memory block, wherein thesecond memory block is coupled to a common source line in the substratethrough the first memory block. The 3D memory device also includes aperipheral circuit configured to apply, to a first set of word lines ofthe first memory block, a preparation voltage based on states of memorycells in the first memory block; float the first set of word lines toinhibit erasing the first memory block; apply an erase voltage to thecommon source line; and apply a low voltage to a second set of wordlines of the second memory block to erase the second memory block.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the present disclosure that others can, byapplying knowledge within the skill of the art, readily modify and/oradapt, for various applications, such specific embodiments, withoutundue experimentation, and without departing from the general concept ofthe present disclosure. Therefore, such adaptations and modificationsare intended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the disclosure and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description and not of limitation, suchthat the terminology or phraseology of the present specification is tobe interpreted by the skilled artisan in light of the disclosure andguidance.

Embodiments of the present disclosure have been described above with theaid of functional building blocks illustrating the implementation ofspecified functions and relationships thereof. The boundaries of thesefunctional building blocks have been arbitrarily defined herein for theconvenience of the description. Alternate boundaries can be defined solong as the specified functions and relationships thereof areappropriately performed.

The Summary and Abstract sections can set forth one or more but not allexemplary embodiments of the present disclosure as contemplated by theinventor(s), and thus, are not intended to limit the present disclosureand the appended claims in any way.

The breadth and scope of the present disclosure should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

What is claimed is:
 1. A method for erasing a selected memory block of athree-dimensional (3D) memory device, wherein the selected memory blockis stacked on an unselected memory block on a substrate and the selectedmemory block is coupled to a common source line in the substrate throughthe unselected memory block, the method comprising: applying, to a firstset of word lines of the unselected memory block, a preparation voltagebased on states of memory cells in the unselected memory block; floatingthe first set of word lines to inhibit erasing the unselected memoryblock; applying an erase voltage to the common source line; and applyinga low voltage to a second set of word lines of the selected memory blockto erase the selected memory block.
 2. The method of claim 1, whereinthe applying the preparation voltage comprises: applying a firstpreparation voltage to the first set of word lines when the memory cellsof the unselected memory block are in programmed states.
 3. The methodof claim 2, wherein the applying the first preparation voltagecomprises: applying a voltage in a range between 0 V and 1 V.
 4. Themethod of claim 2, wherein the applying the preparation voltage furthercomprises: applying a second preparation voltage to the first set ofword lines when the memory cells of the unselected memory block are inerased states, wherein the second preparation voltage is larger than thefirst preparation voltage.
 5. The method of claim 4, wherein theapplying the second preparation voltage comprises: applying a voltage ina range between 1 V and 7 V.
 6. The method of claim 4, wherein theapplying the second preparation voltage comprises: programming a dummydata pattern to the unselected memory block.
 7. The method of claim 1,wherein the applying the low voltage comprises: applying a voltage in arange between 0 V and 1 V.
 8. The method of claim 1, wherein theapplying the erase voltage comprises: applying a voltage in a rangebetween 15 V to 25 V.
 9. The method of claim 1, further comprising:applying the erase voltage to bit lines on the selected memory block.10. The method of claim 1, further comprising: floating bit lines on theselected memory block.
 11. The method of claim 1, further comprising:prior to floating the first set of word lines, applying a voltage of 0 Vto the first set of word lines and the second set of word lines; andsubsequently floating the first set of word lines and the second set ofword lines.
 12. An erasing method for a three-dimensional (3D) memorydevice having a plurality of memory cells vertically stacked on asubstrate, the erasing method comprising: applying a first preparationvoltage to a first word line corresponding to a first memory cell,wherein the first memory cell is at a programmed state; applying asecond preparation voltage to a second word line corresponding to asecond memory cell, wherein the second memory cell is at an erasedstate; floating the first word line and the second word line to inhibiterasing the first memory cell and the second memory cell; applying a lowvoltage to a third word line of a third memory cell to erase the thirdmemory cell, wherein the third memory cell is stacked on the firstmemory cell and the second memory cell; and applying an erase voltage toa common source line shared by the first memory cell, the second memorycell and the third memory cell.
 13. The erasing method of claim 12,wherein the applying the first preparation voltage comprises applying avoltage in a range between 0 V and 1 V.
 14. The erasing method of claim12, wherein the applying the second preparation voltage comprisesapplying a voltage in a range between 1 V and 7 V.
 15. The erasingmethod of claim 12, wherein the applying the low voltage comprisesapplying a voltage in a range between 0 V and 1 V.
 16. The erasingmethod of claim 12, wherein the applying the erase voltage comprisesapplying a voltage in a range between 15 V to 25 V.
 17. The erasingmethod of claim 12, further comprising: prior to floating the first wordline and the second word line, applying a voltage of 0 V to the firstword line, the second word line and the third word line; andsubsequently floating the first word line, the second word line and thethird word line.
 18. A three-dimensional (3D) memory device, comprising:a first memory block disposed on a substrate; a second memory blockstacked on the first memory block, wherein the second memory block iscoupled to a common source line in the substrate through the firstmemory block; a peripheral circuit, configured to: apply, to a first setof word lines of the first memory block, a preparation voltage based onstates of memory cells in the first memory block; float the first set ofword lines to inhibit erasing the first memory block; apply an erasevoltage to the common source line; and apply a low voltage to a secondset of word lines of the second memory block to erase the second memoryblock.
 19. The 3D memory device of claim 18, wherein the peripheralcircuit is further configured to: apply a first preparation voltage tothe first set of word lines when the memory cells of the unselectedmemory block are in programmed states.
 20. The 3D memory device of claim19, wherein the peripheral circuit is further configured to: apply asecond preparation voltage to the first set of word lines when thememory cells of the unselected memory block are in erased states,wherein the second preparation voltage is larger than the firstpreparation voltage.